1. Field of the Invention
The present invention relates to an amplifier circuit in which all of the advantages of power MOSFETs are realized without compromise. It is directed to an amplifier circuit of the common source push-pull output stage with MOSFETs and with enhanced efficiency based on support controlling components for high-frequency and low-frequency signals.
2. Information Disclosure Statement
Audio amplification has been evolving for decades and, with the advent of newer and better electronic components, improved amplifiers move in the direction of what sounds to the human ear to approach perfection. Nonetheless, various new problems arise from enhanced designs and in the inclusion of newly developed electronic components. The following background information and prior art is exemplary of the evolution of audio amplifiers to the present day.
Linear power amplifiers, such as those used in audio amplifiers, traditionally employ an output stage of what is known as the push-pull configuration. In such a configuration, a pair of semiconductor devices is arranged in a series fashion between a positive power supply feed and a negative power supply feed. The point at which the two semiconductor devices adjoin serves as the output terminal. The semiconductor devices are then driven so as to alternately conduct, thus causing the potential at the output node to rise and fall between the two power supply connections.
MOS transistors (Metal Oxide Semiconductor) are frequently used in a push-pull linear output stage. The MOS device has three electrodes: a gate terminal for receiving a controlling signal, a source terminal for establishing a return path for said controlling signal, and a drain terminal for establishing a conductive path to the source terminal; the degree of conductivity being controlled by said controlling signal. Significant quantities of capacitance exist between these electrodes; the controlling signal must charge and discharge these capacitances to modulate the conductivity of the path existing between the source and drain electrodes.
Returning to the discussion of the push-pull linear output stage, a MOS device of a positive type (PMOS) and a MOS device of a negative type (NMOS) are generally used; the contrary nature of their electrode polarities facilitate implementation in the push-pull configuration. There are two ways of arranging this connection. One is the "source follower" configuration. In this implementation, the source terminals of the respective PMOS and NMOS transistors are interconnected to form the output node. Positive power is fed to the drain terminal of the NMOS device, and negative power is fed to the drain terminal of the PMOS device. The chief drawback to this configuration is that as the output potential is modulated the source terminals "follow" the modulation, hence upsetting the potential difference between the source and gate terminals. Such a configuration requires that the drive signal potential exceed the desired output potential in both the positive and negative direction. This is necessary to insure that sufficient potential difference exists between the gate and source terminals to sustain conductivity from drain to source.
The advantage of this configuration is that the above-described "bootstrapping" phenomenon lessens the burden which the interelectrode capacitances pose on the driver stage. Another benefit is that the potential existing between the gates of the respective MOS devices is of a relatively small magnitude, making it easy to establish a single path to both gates; a necessary step in completing the circuit architecture. Hence, speed and linearity are enhanced.
The second way of arranging the push-pull connection is known as the "common source" configuration. In this implementation, the drain terminals of the respective PMOS and NMOS transistors are interconnected to form the output node. Positive power is fed to the source terminal of the PMOS device, and negative power is fed to the source terminal of the NMOS device. The bootstrapping phenomenon exhibited by the source follower arrangement is eliminated, because excitation of the common source output stage does not cause a corresponding disruption of the source terminal potentials of the respective MOS devices. Hence, relatively small signal potentials are sufficient to cause large output modulations.
The chief drawback is that the interelectrode capacitances of the MOS devices are not bootstrapped, and pose a significant burden on the driver stage. Furthermore, a large potential exists between the gate terminals of the MOS devices, making it difficult to establish a single path to both gates. These drawbacks tend to compound one another, since the driver stage is burdened by both the current requirement and the potential difference between the MOS gates. Consequently, speed and linearity are diminished.
U.S. Pat. No. 4,464,634, issued to Juan F. Velezquez, describes an "Audio Power Amplifier", which utilizes an input amplifier stage and an output amplifier stage. The output power amplifier stage utilizes a P-channel MOSFET and an N-channel MOSFET having interconnected drain electrodes forming an output signal terminal. A driver circuit is provided which operates in a transconductance mode wherein the output current through the MOSFETs is directionally proportional to the input voltage applied to the output amplifier stage. Feedback paths are used between the output power amplifier stage and the input amplifier stage as well as between the output and input of the input amplifier stage.
U.S. Pat. No. 4,467,288, issued to James C. Strickland, describes an amplifier utilizing an input amplifier, an output amplifier and a cooperative combination of negative and positive feedback loops that removes substantially all distortion and error in the amplified output, according to the inventor. Negative feedback loops are provided around the output amplifier and the input/output amplifier combination to provide degenerative error reduction. The negative feedback loop around the MOSFET output stage identifies the error component and inversely superposes an inverted signal. In other words, the precorrection is precisely controlled to effect virtually complete serial cancellation of the to-be-introduced error as the signal is propagated through the amplifier.
U.S. Pat. No. 4,483,016, issued to Peter A. Hochstein and Kelvin Shih, describes an "Audio Amplifier", using a differential amplifier stage, a primary amplification stage, bipolar supply connections for supplying electric current, the primary amplification stage including at least two MOSFETs in common source connection between the power supply connections and a circuit transducer output, along with a level shifting driver stage connected to ground.
U.S. Pat. No. Re. 31,749, which is a reissue of a U.S. Pat. No. 4,100,502, issued to Osamu Yamashiro, describes an amplifier circuit with a complementary inverter which includes a P-channel metal-insulator-semiconductor FET connected to a first source potential, an N-channel metal-insulator-semiconductor FET connected to a second source potential with the gate of the two FETs with a common linear input, respective load resistors connected to the drains of the FETs, an output being derived from the interconnection of the load resistors or from the drains of the FETs, and a bias resistor between the gate and the drain of each of the complementary FETs, the input being supplied to the gate of the FETs through respective capacitors. The FETs are individually biased so that the circuit may serve as a Class B push-pull amplifier of low power consumption.
U.S. Pat. No. 4,743,860, issued to James M. Dziagwa, sets forth a power amplifier circuit having a first and second series of capacitors with a junction between them which are coupled across a floating, double ended power supply. A speaker is coupled between the junction and the ground and first and second complementary conductivity power MOSFETs are connected between a first power supply output and a ground and between a second power supply output and a ground, respectively. The MOSFETs are driven by an amplified audio input signal and alternatively connect to one or the other supply output to ground to alternately charge and discharge the capacitors, causing alternating current to flow through the speaker.
An article in Volume 63, Number 4 of Radio Electronics., April, 1992, entitled "High Power Hi-Fi Audio Amp For Your Car", at Page 31 et seq., describes an audio amplifier in which output swings of an operational amplifier drive a current source and a current sink for the purpose of translating and applying said output swings to an NPN transistor and a PNP transistor, respectively. The voltage drop between the bases of said NPN and said PNP transistor are determined by a voltage multiplier. FET output transistors are then driven into conduction by said NPN transistor and said PNP transistor.
Notwithstanding the above prior art and the state-of-the-art as understood, it appears that the present invention concept wherein capacitors are used as a high-frequency signal path and a separate path is provided for the low-frequency signal component with D. C. bias, by its own circuit, is neither taught nor suggested.